1. Field of the Invention
The present invention is generally related to light emitting diodes and, more particularly, to a light emitting diode made with a doped gallium arsenide substrate on which a gallium arsenide epitaxial layer is deposited.
Description of the Prior Art
Silicon-doped gallium arsenide light emitting diodes are typically grown on silicon-doped gallium arsenide substrates. Depending on the precise process conditions employed during the various manufacturing processes, the surface of the N-type silicon-doped substrate can convert to a P-type layer which results in deleterious thyristor-like behavior. This type of behavior results in the light emitting diode being useless. This problem is wide spread in the industry and is typically exacerbated when the substrate is heated beyond certain temperatures.
The cause of the P-type conversion is related to the fact that silicon, which is proximate the surface of the substrate, changes from the gallium sites to the arsenic sites during the high temperature saturation of the melts. This effect is caused primarily by the high number of arsenic vacancies which are caused as the arsenic boils off from the substrate at elevated temperatures. The large quantity of arsenic vacancies drives the silicon to the arsenic sites. Silicon, on an arsenic site, is an acceptor and, therefore, a layer of P-type conductivity material is created at the surface of the substrate. Silicon is the dopant in the substrate and is uniformly distributed. The chemical concentration of the silicon is approximately 1.sup.* 1020 atoms per cubic centimeter. Silicon is distributed on both arsenic and gallium sites. In melt grown silicon doped gallium arsenide, which is grown at approximately 1240 degrees centigrade, the silicon atoms are distributed so that there is a net donor concentration of approximately 1*10.sup.18 electrons per cubic centimeter. The site distribution for the silicon in gallium arsenide is a function of the growth method, the temperature, the growth conditions and time. A condition that is at equilibrium as 1240 degrees centigrade is not necessarily the equilibrium condition at 100 degrees centigrade. The material will attempt to achieve equilibrium, but the rate at which it achieves equilibrium can vary significantly. One problem that can occur when a slice of gallium arsenide is heated is that the more volatile constituents may evaporate from its exposed surfaces. In the case of a slice of gallium arsenide, arsenic is more volatile than the other constituents in the material (i.e. gallium and silicon). Therefore, arsenic will evaporate from the surface of a slice of gallium arsenide. The amount and rate of evaporation is a function of temperature. The rate increases greatly at higher temperatures. When arsenic leaves the surface of the gallium arsenide, empty spaces remain in the gallium arsenide slice's crystal lattice. The empty spaces are arsenic vacancies. The arsenic vacancies can either stay where they exist or diffuse into the bulk of the gallium arsenide slice. The rate at which they diffuse is a function of temperature and the concentration of the arsenic vacancies. The excess arsenic vacancies create a condition in which the silicon atoms are in a nonequilibrium state with respect to the crystal lattice and some silicon atoms in the vicinity of the excess arsenic vacancies change sites from either interstitial or gallium to the arsenic site. This site change results in a net donor decrease and more P-type conductivity material. The site changing is also a function of temperature and constituent composition. When sufficient silicon atoms change sites, the gallium arsenide material can change from N conductivity type material to P conductivity type material.
In an article titled "High-Efficiently Graded-Band-Gap Ga.sub.1 -.sub.X Al.sub.X As Light-Emitting Diodes" by L. Ralph Dawson in the June 1977 edition of the Journal of Applied Physics, Volume 48, Number 6, describes the use of a gallium aluminum arsenide epitaxial layer in which an LED is manufactured by silicon-doping the gallium aluminum arsenide epitaxial layer. This material is described as being superior to gallium arsenide which is silicon-doped. This article describes the preparation of a graded gallium aluminum arsenide epitaxial layer on a gallium arsenide substrate. In the process, the author describes the substrate as being etched away and the device uses only the grown gallium aluminum arsenide material that remains. Since the substrate is etched away during this process, if there was a surface type conversion occurring in the material, it was removed and thus does not become a factor in the performance of the device. As is known to those skilled in the art, the NPNP condition can also be averted by etching the substrate immediately before putting the epitaxial layer on it in a reactor.
The problem described above results in a P-type conductivity region on the upper surface of the substrate. When the epitaxial layer is grown on the upper surface of the substrate, the initial epitaxial growth process results in an N-type conductivity type region within the epitaxial layer immediately proximate the upper surface of the substrate. Following the deposition of that N-type conductivity region, the remaining portion of the epitaxial layer is of P-type conductivity material. The existence of the P-type conductivity region at the upper portion of the substrate, proximate its upper surface, results in the creation of an NPNP device that exhibits thyristor-like behavior. Naturally, this type of behavior in a light emitting diode structure is severely disadvantageous and results in an unusable device that must be scrapped. This situation is exacerbated by the fact that this behavior is not generally discovered until the light emitting diode is completely manufactured and tested. It would therefore be significantly beneficial if a means is provided to prevent the creation of the P-type conductivity region proximate the surface of the substrate in response to elevated temperatures.